Acoustic attenuator



May 17, 1949. J. G. wooDwARD ACOUSTIC ATTENUATOR Filed Jan. 51 1946 ATTORN EY Patented May 17, 1949 ACOUSTIC Ar'rENUA'roR J. Guy Woodward, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application January 31, 1946, Serial No. 644,525

7 Claims.

This invention relates to improvements in acoustic attenuators used with wind instruments. More specifically, it relates to an improved attenuator or mute to be used with cornets or trumpets although the device could be used with other wind instruments such as the trombone.

In general, the idea of using a mute to either distort the tone or cut down the intensity of sound output of a trumpet is old. Hundreds of varied shaped mutes have been constructed, most of them to give some unusual distortion effect to the sound output of the instrument. Almost none of the mutes commercially obtainable has been constructed on any scientific design.

The preferred type of mute embodying the present invention is not the -distortion type but rather one which attenuates the sound as uni-- ii'ormly as possible over a wide range of frequencies. This type of mute is useful for achieving jproper balance between the various instruments Iand sections of an orchestra, especially when playing before microphones. The problem of balance in this connection has been one of major concern to program producers, musicians, and broadcast engineers alike due to the high level of 'sound produced by the brass instruments, particularly the trumpets, compared with the rest of the orchestra. The problem cannot be solved by merely having the trumpets play softly since, in the first place, the trumpet loses much of its characteristic quality when played softly and, second, the musician finds it more difficult to control tone and pitch.

It is an object of the present invention to provide an attenuator for wind instruments which has been designed on scientific acoustic principles.

Another object is to provide a mute for instruments such as the cornet and trumpet which attenuates the sound output uniformly over a 'wide range of frequencies.

Another object is to provide an acoustic attenuator for wind instruments which is especially adapted to attenuate a certain band of frequencies.

A further object is to provide an acoustic attenuator which cuts down the sound output uniformly without distorting the tone to any appreciable extent.

A still further object is to provide an attenuator which, although being designed in accordance with scientific acoustic principles, is 'still of simple construction and light Weight.

Other objects and advantages of the invention -will become apparent from a further description of the invention including the drawings in which:

Fig. 1 is a rear perspective View of a preferred embodiment of the attenuator.

Fig. 2 is a front View of the attenuator shown in Fig. 1.

Fig. 3 is a cross-sectional view of the attenuator shown in Fig. 1.

Fig. 4 is a cross-sectional View of another modification of the attenuator.

Fig. 5 is a cross-sectional view of still another modification of the attenuator.

Fig. 6 is a typical attenuation curve compiled from data resulting from operation of the attenuator similar to the one illustrated in Figs. 1 and 2.

Fig. 7 is another attenuation curve.

The device is preferably made of aluminum spinnings in two sections I and 2 forming a cavity 3 between them. The volume (acoustic capacitance) of the cavity combined with the narrow opening (acoustic inertance) at yIl provides a system which is resonant at a desired, convenient frequency. When the attenuator is inserted in the bell 5 of an instrument, the sound leaving the instrument and passing `through the mute is partially absorbed by the resonant cavity, the absorption being greatest at the resonant frequency. A band of cork 6, or other material having a high coefficient of friction, aids in holding the attenuator in the bell of the horn. In the instance of an attenuator designed for trumpets or cornets, it is preferred to have a system which resonates at a comparatively low frequency such as 300 cycles per second. This may be obtained by using a cavity having a volume of 12,4 cubic inches and a separation of approximately inch between the iiaring Walls at their small ends. By varying the volume of the cavity in relation to the size of the opening, this resonant frequency can be changed as desired. For cornets and trumpets, the low resonant frequency is desired so that the attenuation may be obtained for the low frequency range of notes.

In the design as illustrated, the resonant cavity has gradually flarin'g walls. This causes the acoustic constants, capacitance and inertance, to be distributed rather than lumped and results in a resonant system which will allow absorption of sound by the cavity to occur over a fairly wide band of frequencies.

It is also possible to have an attenuator of conic shape although this form does not have as desirable distribution of the acoustic constants as the form which has flared walls.

In the design of the attenuator shown in Figs.

1-3 and which will now lbe described by way of example, the attenuation of frequencies below 300 C. P. S. becomes increasingly less, the lower the frequency. However, the lowest note ordinarily played on a trumpet has a frequency of 165 C. P. S. and notes below 350 C. P. S. cannot usually be played so loudly as to require attenuation. To make the band of absorption at low frequencies still broader and less dependent on frequency, additional acoustic resistance can be inserted in the narrow opening of the cavity atl. One form which this resistance may take is .a number of small aluminum blocks I inserted side by side in the narrow opening `with small open spaces between blocks. For example, in the resonant system shown, the blocks may have dimensions of 578 inch long .by .agg inch 'Wide `by 0.03 inch thick. Thus, the sound must enter the cavity through a number of slots which give the desired resistance, resistance being varied by varying the spacing between blocks. .Another way-of accomplishing'the same result is to wrap several layers of cloth about the inner section of the attenuator at the narrow opening.

The effect of the resonant cavity in attenuat- VYing the sound is .restricted almost entirely to frequencies below 1,000 C. P. S. To get attenua- -tion at higher frequencies, additional lacoustic resistance 8 in thefor-m of several layers of lcloth `is placed at the large .endof the attenuator. VThe cloth used may besilk, nylon-or similar material and the layers are mounted between two nemesh metal screens 9 and I0 supported around .the edge bya metal ring Hand screws or rivets. The effect of the cloth layers, in attenuating the sound, increases with the frequency and is .a1- .most negligible below A1,000 C. P. 2S, compared to .its Aeffect at higher `frequencies and tothe effect of the cavity. Hence, by :securing the proper .relation .between 4the v.two .types of absorption, attenuation can be .achieved lwhich is .quite infdependent of frequency over nearly the whole .range `of yfrequencies present in the tone .of the trumpet.

rIfypical.atter-iuati-on .curves are Ashown .in Figs. vfi-and v'7. These .curves are the result of plotting the differences between frequency response curves .of a trumpet `Without and with the mute inserted vin its bell. I n taking the .data from .which the response `curves were drawn, the trumpet was .driven :by .a magnetic loud `speaker unit coupled to itat the mouthpiece ,end of Vthe horn. "The remainder of the testprocedure .and .equipment is the same .as that `commonlyused for loudspeakers.

.As shown .in the curves, Lthe attenuation .decreases markedlyV below .300.C..P S..and, as shown particularly in Fig. 7, may v.actually lbecome nega.- it'ive .below 200 '.C. P. lS. This situation could be .remedied by using a `cavity hav-ing a .larger volumesince theresonant frequency of the .cavity decreases as y.the cavity is increased in size. .It is undesirable, however, to make the .dimensions of the attenuator `any .larger than necessary. Furthermore, as pointed .out vpreviously, l,the notes vbelow 350 C. P. S. cannot ordinarily .be played loudly venough to require attenuation. Hence, modifications L-for .attenuating these frequencies muy be omittedif desired.

It may also be noted from ,the .curves thatvthe degree of attenuation varies ...considerably at frequencies above 1,000 P. S. This .can be .eliminated .by mounting .within .the :attenuatorsmall vresonators (not shown) -tuned to the-frequencies of .the ,peaks in .the response curve. .But listening tests indicate this additional precaution t0 be unnecessary for most purposes since variations of i2 decibels in the attenuation curve above 1,000 C. P. S. do not seriously alter the quality of the trumpet tone providing the variation does not include too wide a range of frequencies.

From the foregoing description, it will be seen that the uniformity of attenuation `depends upon the relative values of the acoustic Vcapacitance and inertance as well as upon the amount of acoustic resistance used in the attenuator. Acoustic capacitance is analogous to electrical capacitance in electrical circuits. Acoustic inertance is analogous to electrical inductance While acoustic resistance is akin to electrical resistance. By properly adjuting the relative values of resistance, capacitance and inertance, there is obtained a system which attenuates a wide band of frequencies including most of the range normally played on the instrument. 'Ihe `preferred attenuation .design is found. for any particular wind instrument, by selecting a cham,- yber having the proper ratiobetween sizeof cavity .and size of .openingI and the US@ Qf gradually .aring walls enclosing the cavity ,and its .openE ine helps to provide more uniform response over a usable band of frequencies.

There has thus vfar been .described and `eniphasized a form of .mute which ,attenuates .u wide band of frequencies with ya high degree of uniformity. Without departing from the basic Aconcept of the invention, there may also be designed mutes which will attenuate Some special range of frequencies such as those in the higher or lower region. The desired attenuation may be obtained by balancing the capacitance, inertance yand resistance .so as to obtain .the .desired result. For-example, by making .the ca -incitan ce higher or lower, which is to say, making the lvolume `of the cavity greater or less, the principal reso-.- nant frequency of the system .can be made lower -;or higher. The greater the volume of ,the cavity, the lower is the resonant yfrequency and there.- fore the lower ,thefrequency Vof `greatest attenuation. As previously pointed out, .the lsize of .the openings into the cavi-ty determines .the inert.- ance. If itis desired to keep thesizeof the-.cavity constant, the resonant frequency may be varied b y changingthesizeof theopenings. By lincreasing the size r.of .the openings, v.the reso? nant frequency lis made higher .and Vie versa.

No general rule can .be .stated concerning the use of resistance in the attenuatng system. .Usuallythose materials which .are the. best sound .absorbers .such as cloth, .Celotex, etc., .are 3,150 the best attenuators of the higher frequencies. The use of very small openings into the resonat- .ing-cavity can be'made to ,supply resistancewhich will Aattenuate vthe .lowest `frequenuies Since, `in the .case lof rnarrow `openings, their resistance Value is more important than their ,inertance value.

Modiiicai-.ions inthe .design of the attenuator `may also be `made `as illustrated in ,lF g,S.-4 rand. .Inthe form .shown in Fig. 4, .the-cavity I2 hasta capacitance depending on volume. Obstruc- .tions I3 s uch as aluminium blocksmayibe Jplaced in the Inarrow :opening `o f-the .cavity to provide both some desired resistance inertance. Additional resistance in the form of laiels cof cloth .I4 may be `placed 4.ina'frame ,4.5 to attenuate yhigher frequencies.

Fig. 5 illustrates -a `modifloat-ion similargto 3 but Inavinstlle openings :I6 ...nto fthe cavity Il .located along'the-fla'red :Walls instead ,of at the end. Resistance I8 may lbe-.finseited .Within the cavity and additional resistance I9 such as layers of cloth may be placed over the mouth of the attenuator opening. In this type of design, it is preferable to have openings I6 consist of a number of perforations to provide the desired inertance.

I claim as my invention:

1. An acoustic attenuator for a wind instrument comprising a resonant cavity having one end insertable within the bel1 of said instrument, an opening adjacent said insertable end and means having relatively high acoustic resistance positioned Within said cavity adjacent said Opening.

2. An attenuator, according to claim 1, in which said means having acoustic resistance comprises a plurality of small metallic blocks uniformly spaced around said opening so as to form a number of narrow slots.

3. An attenuator according to claim 1 in which said cavity has a gradually increasing diameter in a direction away from said opening.

4. An attenuator, according to claim 3, in which said resistance comprises a plurality of small aluminum blocks evenly spaced around said opening so as to form a number of narrow slots.

5. An acoustic attenuator for a wind instrument comprising, in combination, a resonant cavity having a resonant frequency below 1,000 cycles per second adapted to be inserted within the bell of said instrument, means having rela tively high acoustic resistance positioned within said cavity and sound absorbing means for attenuating audible frequencies above 1,000 cycles per second adapted to be positioned in acoustic relationship with said bell.

6. An attenuator, according to claim 5, in which said sound absorbing means comprises a plurality of layers of cloth.

7. An acoustic attentuator for a wind instrument comprising a resonant cavity of annular configuration surrounding a central air passage, said cavity having a narrow tapered end and a ared end, said narrow end being adapted to iit within the bell of said instrument and having an opening adjacent thereto, means for attenuating audible frequencies below 1,000 cycles per second positioned within said cavity adjacent said opening, and means for attenuating audible frequencies above 1,000 cycles per second positioned across said air passage.

J. GUY WOODWARD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,338,108 Sordillo Apr. 27, 1920 1,342,846 Emma June 8, 1920 1,697,707 Berg Jan. 1, 1929 1,702,561 Emma Feb. 19, 1929 2,244,205 Koeder June 3, 1941 2,318,535 Spivak May 4, 1943 

